1. Field of the Invention
The invention relates generally to cryostats and more particularly to cryostats that include detectors that operate at very low temperatures.
2. Description of the Related Art
A cryostat is an apparatus that provides a low temperature working environment required for devices that operate at very low temperatures. A typical cryostat includes a vacuum vessel that encloses a detector such as a measurement instrument, that operates at very low temperatures. A cryostat also includes a cooling mechanism for cooling the detector.
Frequently, it is desirable to remove from a cryostat vacuum vessel residual gases, such as water, oxygen, carbon dioxide and nitrogen, for example. Such residual gases often can contaminate a detector and degrade its performance. In particular, at reduced temperatures, residual gas tends to be cryoabsorbed by cold surfaces within the vacuum vessel. Cryoabsorption is a phenomenon whereby a warmer gas tends to be absorbed by a cooler surface. Such cryoabsorption on surfaces of a detector within a cryostat vacuum vessel often can have deleterious effects upon operation of such a detector. For example, germanium detector diodes used for detecting high energy photons in the form of X-rays and gamma-rays must be virtually free of surface contaminants in order to avoid leakage current which degrades detector performance. Cryoabsorption of residual gases by such diodes causes surface contamination which degrades their performance. Moreover, it should be noted that a detector, such as a germanium diode, for example, can be damaged through contamination to the extent it must be removed from the cryostat for reprocessing before it can be used again.
In the past, residual gas in a vacuum vessel of a cryostat has been removed, for example, by employing a cryoabsorb such as activated Zeolite or activated charcoal which at low temperatures, readily absorbs the residual gas. A typical cryoabsorb provides a relatively large bonding surface for the residual gas, and at low temperatures, such cryoabsorbs remove the residual gas through bonding ("absorption") of the residual gas by such cryoabsorbing bonding surfaces. Thus, in the ideal case the residual gas is cryoabsorbed by the cryoabsorb instead of by a detector within the vacuum vessel.
While earlier cryostats generally have been acceptable, there have been problems with their use. For example, while cryoabsorbs can successfully absorb residual gases, they also tend to deabsorb these gases when temperatures increases again. Furthermore, typically, as temperatures within the vacuum vessel increase, the detectors heat up the most slowly. Consequently, there is a risk that as the temperature increases, the deabsorbed residual gas will be recryoabsorbed by a relatively cool detector resulting in contamination of the detector surface.
One earlier solution to this problem was to pump out such deabsorbed residual gas during such temperature increases so that it could not be recryoabsorbed by a detector within the vacuum vessel. Another solution was to coat a detector, such as a germanium detector diode, with an electrically inert material such as silicon monoxide, for example, that could inhibit the effect of cryoabsorbed residual gas. Neither of these solutions has been totally satisfactory. The first generally requires the use of equipment such as an external vacuum pump to remove residual gas from the vacuum vessel of a cryostat. Often such a pump cannot be conveniently transported to remote locations where the cryostat can be used. The second often still results in unacceptable degradation of detector performance.
Still another problem involved cryostats that employed devices such as germanium detector diodes to detect physical phenomena such as high energy photons. More specifically, germanium diodes of the type mentioned above, for example, not only must be virtually free of surface contamination but also must have virtually perfect crystal lattice structures in order to perform satisfactorily. In operation, however, the bombardment of the germanium crystal lattice by high energy particles can cause damage to the lattice structure which can result in degradation of the ability of the diode to detect. The lattice structure usually can be repaired through annealing; that is accomplished by raising the temperature of the germanium diode to approximately 400.degree. K. for approximately twenty-four hours.
Thus, there has been a need for an improved cryostat in which temperatures can be raised above low temperature levels without undue risk of contaminating detectors through cryoabsorption of residual gases on the surface of the detectors. Furthermore, there has been a need for such a cryostat in which a device, such as a germanium detector diode, for example, can be annealed in situ without the need to attach an external pump to the cryostat. The present invention meets these needs.